In presently known equipment of this type, the armature of the magneto generator also constitutes the ignition coil. The primary winding of the ignition coil is short-circuited by the emitter-collector circuit of a transistor connected in a Darlington configuration until the ignition time. Because of this short-circuit, the voltage in the primary circuit has a very small amplitude and is almost in phase with the primary current. To initiate ignition, a threshold circuit is used which, when a predetermined threshold voltage is exceeded in the primary circuit, generates a signal blocking the above-mentioned transistor. The magnet wheel of the magneto generator generates both positive and negative half waves in the primary circuit. If the negative half wave is not utilized for ignition, it is damped by a damping circuit so that the no-load voltages in the primary circuit cannot reach too high a value. It is the disadvantage of these known ignition systems that no speed limiting is provided, so that high speed tools such as, for example, saws or grinders may be destroyed by excessive speed.
In German published application No. 2 405 382 a speed limiting circuit is disclosed which, when the maximum speed of the internal combustion engine is passed, causes the ignition transistor to be short-circuited during the positive half wave, that is the half wave which is utilized for ignition. This type of speed limiting has the disadvantage that ignition is abruptly switched on and off. This can cause undesired sparks to be generated in the crank shaft housing and in the exhaust system, thereby causing the internal combustion engine to be destroyed. Further, the power of the engine is abruptly switched in and out, which may lead to undesired oscillations.